Denver-based Frontier Airlines announced Wednesday it would begin charging passengers who book tickets on third-party web sites -- such as Expedia and Orbitz -- for bringing their bags onboard the flight. Passengers who purchase tickets on Frontier's web site will incur no additional charge for a carry-on bag.

The cost of a carry-on for passengers booking on third-party sites is $25 - $100. The fees will go into effect this summer, the carrier said, with the date to be announced soon.

All customers are allowed one free personal item, such as a briefcase or purse, for free. The personal item must fit in under the seat.

The move to charge passengers for carry-on bags mimics that of Spirit Airlines, previously the only domestic carrier to charge for carry-on bags. The carrier also charges between $25 and $100 for a carry-on; however, the charge extends to all passengers regardless of where the ticket is booked.

"Frontier's most loyal customers have made it very clear that finding overhead bin space for carry-on bags has become unacceptably difficult. In response, Frontier will be introducing a charge for carry-on bags for customers buying Basic fares through third party sites," the carrier said.

The move is designed to incentivize customers to book directly with the airline. Every time a passenger books through a third-party site, the carrier has to pay a commission to the agency. But enticing customers to book directly with the airline also makes comparison shopping more difficult for the consumer. Frontier has been a leader in this fight, announcing in September only passengers who book direct with the airline are able to reserve seats in advance.

Frontier will also begin charging $1.99 for coffee, tea, soda and juice on July 1 to customers who purchase economy or basic fares. Spirit Airlines also charges for soft drinks. In 2009, US Airways implemented a $2 charge for soft drinks, a move that was quickly rescinded because of its unpopularity with customers.